Abstract
Abstract. This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.
Highlights
Large-scale thrusting and nappe stacking are the main processes responsible for crustal shortening during continental collision
The Eide melting event is dated at 702 ± 5 Ma, indicating that it is of pre-Caledonian age (Gasser et al, 2015), and Caledonian migmatization has not been recorded in the Kalak Nappe Complex (KNC) so far
The relative positions and relationships between the different nappes in the Late Ordovician and Silurian suggests that the Vaddas and Kåfjord sediments were deposited in a continental basin prior to early Silurian heating
Summary
Large-scale thrusting and nappe stacking are the main processes responsible for crustal shortening during continental collision. Hollister and Crawford, 1986; Beaumont et al, 2006; Gerya and Meilick, 2010; Labrousse et al, 2010; Jerábek et al, 2012). Based on its similarity to rocks at the same tectonostratigraphic level (e.g. the Magerøy Nappe, located in northern Finnmark; Andersen et al, 1982; Corfu et al, 2006), the RNC is considered equivalent to other Iapetus-derived or outer Baltica margin (upper allochthon) units in the Caledonides It may preserve an early Silurian history, recording events immediately prior to or during early continental collision between Baltica and Laurentia (e.g. Andréasson et al, 2003; Slagstad and Kirkland, 2018). The pre-Caledonian and Caledonian evolution of the rocks affected the deformation behaviour during nappe stacking in this particular part of the Caledonian orogen
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